1. Field of the Invention
The present invention relates to a novel cyclic pyrazolo compound, 3-(.beta.-D-ribofranosyl)pyrazolo[3,4-e][1,3]oxazine-5,7-dione (hereinafter briefly referred to as pyrazolo[3,4-e]oxazine C-nucleoside), represented by the following formula: ##STR1## and a process for preparing the same.
2. Discussion of the Background
Pyrazofurine (pyrazomicine) is a C-nucleoside isolated from a liquid culture medium of Streptomyces candidus and represented by the following formula: ##STR2## The pharmaceutical properties of the aforementioned pyrazomicine have attracted general attention in recent years. Further details of the compound and the pharmaceutical properties thereof will be found, for example, in G. D. Daves Jr., et al, Prog. Med. Chem. 13, 303(1976); and K. Gerzon et al, the Second International Congress of Heterocyclic Chemistry (ICHC), Collection of Summary of Reports, page 131(1969). These articles will be incorporated herein as references.
However, only three synthetic processes, other than the biochemical culture method, have been reported up to now. These three preceding synthetic processes are disclosed in J. Farkas et al, Tetrahedron Letters, 1972, 2279; S. De Bernardo et al, J. Org. Chem., 41, 287 (1976); and J. G. Buchanan et al, J. Chem., Soc. Perkin I. 2374 (1981).
However, these known processes have defects in connection with yield, selection of starting materials and reaction sequence, making it desirable to provide an improved process.
In view of the aforementioned circumstances, we have already proposed a simplified process for preparing pyrazomicine from 3-alkoxylcarbonyl-2-oxopropylidene triphenyl-phosphorane represented by the general formula ph.sub.3 PCH.COCH.sub.2 CO.sub.2 R. The reaction sequence of our earlier proposed process will be shown below by the reaction formulae, and the details thereof can be found in, for example, Japanese Laid-Open Patent Publication No. 134088/1983, the disclosure of which is incorporated herein as a reference. ##STR3##